The present invention relates to an eddy current testing probe comprising an exciting coil and a detecting coil, for use in detection of surface flaws of a conductive test material.
An eddy current testing probe is used to detect surface flaws of conductive materials and products such as metals. FIG. 1 is a schematic view showing schematically the configuration of a conventional general eddy current testing probe. The conventional general eddy current testing probe comprises an exciting coil 1 in the shape of a circular ring, and a detecting coil 2 in the shape of a circular ring having the same diameter as the exciting coil 1. The exciting coil 1 and the detecting coil 2 are arranged parallel to each other, and a surface of the detecting coil 2 on the side opposite to the exciting coil 1 functions as a flaw detection surface. When using such an eddy current testing probe, the surface of a test material T, such as a conductive material and product, and the flaw detection surface are arranged to face each other with a suitable distance therebetween, the exciting coil 1 and the detecting coil 2 are positioned so that the center axis of the exciting coil 1 is substantially orthogonal to the surface of the test material T, and then an alternating current is caused to flow in the exciting coil 1. As a result, an AC magnetic field is generated around the exciting coil 1, and an eddy current is induced on the surface of the test material T by the AC magnetic field.
If there is a flaw on the surface of the test material T, the eddy current flows along the flaw. Therefore, when the eddy current testing probe is moved from a portion where no flaw is present to a portion where a flaw is present, the path of the eddy current changes. Accordingly, the strength and direction of a magnetic field caused by the eddy current vary, and a voltage between the terminals (output voltage) of the detecting coil 2 induced by this magnetic field changes. Since this voltage change is generally detectable as a change in the amplitude and phase of AC voltage, the amplitude and phase of the voltage between the terminals of the detecting coil 2 are measured, and the presence/absence and properties of flaw on the surface of the test material T are detected from the measured results.
Compared to other eddy current testing device such as a through coil that carries out an eddy current test by inserting a test material into a solenoid coil, an eddy current testing probe as mentioned above is applicable to various shapes of test materials and has a simple structure, and therefore it is used in a variety of fields. In such a conventional eddy current testing probe, however, the output of the detecting coil 2 contains a phase component due to the distance between the exciting coil 1 and the surface of the test material T, i.e., so-called lift-off, and a change in lift-off is detected as a noise component. Therefore, there are disadvantages that it is difficult to detect only a flaw and it is hard to adopt a phase analysis used for analyzing the properties of flaw such as the type and depth of the flaw.
The following description will explain an operational principle of the conventional eddy current testing probe. A voltage Vc between the terminals of the detecting coil 2 is expressed by the sum of a voltage Vex induced by a magnetic field generated by an exciting current Iex flowing in the exciting coil 1 and a voltage Vin induced by a magnetic field generated by an eddy current Iin.
Vc=Vex+Vinxe2x80x83xe2x80x83(1) 
Here, the voltages Vex and Vin can be expressed by equations (2) to (5).
Vex=Axc2x7(dxcfx86ex/dt)xe2x80x83xe2x80x83(2) 
xcfx86ex=Bxc2x7Iex+"PHgr"1(d)xe2x80x83xe2x80x83(3) 
Vin=Cxc2x7(dxcfx86in/dt)xe2x80x83xe2x80x83(4) 
xcfx86in=Dxc2x7Iin+"PHgr"2(d)xe2x80x83xe2x80x83(5) 
where A, B, C, D: constants,
xcfx86ex: the strength of the magnetic field generated by the exciting current Iex,
"PHgr"1(d): a varying component of xcfx86ex due to a change in lift-off d,
xcfx86in: the strength of the magnetic field generated by the eddy current Iin, and
"PHgr"2(d): a varying component of xcfx86in due to a change in lift-off d.
Thus, when the lift-off d changes, since the magnetic fields xcfx86ex and xcfx86in vary accordingly, both of the amplitude and phase of the voltage Vc between the terminals of the detecting coil 2 change.
For such a reason, when the lift-off changes or the angle of the exciting coil 1 to the surface of the test material T changes, there occurs a change in the noise component and the phase component due to lift-off, contained in the output of the detecting coil 2 as described above. Therefore, conventionally, there has been used eddy current testing probes having a structure capable of scanning the surface of the test material T while maintaining constant lift-off, or a structure capable of measuring the amount of lift-off and correcting the output of the detecting coil 2 so as to remove the component due to lift-off from the output. Such eddy current testing probes have the problems of complicated structures and high prices.
In order to solve the problems, the following eddy current testing probe was proposed, and reported at p. 131 of the Abstract of Fall Conference, 2000, of the Japanese Society for Non-Destructive Inspection (hereinafter referred to as the xe2x80x9cprior art referencexe2x80x9d). FIG. 2 is a schematic view showing schematically the configuration of the eddy current testing probe reported in the prior art reference, and FIG. 3 is an explanatory view for explaining the operational principle of the eddy current testing probe. As shown in FIG. 2, this eddy current testing probe comprises an exciting coil 1 in the shape of a circular ring and a detecting coil 2 in the shape of a quadrangular ring, and the exciting coil 1 and the detecting coil 2 are positioned so that the center axis of the detecting coil 2 is orthogonal to the center axis of the exciting coil 1 in the state where one side of the detecting coil 2 is placed in a diameter direction of the exciting coil 1, inside the exciting coil 1.
FIGS. 4A and 4B are explanatory views for explaining the path of the eddy current generated on the surface of the test material T. As shown in FIG. 4A, when there is no flaw on the surface of the test material T, the eddy current on the surface of the test material T flows in a circumferential direction equal to the winding direction of the exciting coil 1. In this case, almost no magnetic field is generated in a direction crossing the detecting coil 2 by the eddy current, and therefore almost no electromotive force is generated in the detecting coil 2. Further, in this case, since the output of the detecting coil 2 is substantially zero, even when lift-off changes, the output of the detecting coil 2 contains almost no noise component due to the change in lift-off.
On the other hand, as shown in FIG. 4B, when there is a flaw on the surface of the test material T, the eddy current flows along the flaw. When the detecting coil 2 is parallel to the longitudinal direction of the flaw, as shown in FIG. 3, a magnetic field is generated in the direction crossing the detecting coil 2 by the eddy current, and an electromotive force is generated in the detecting coil 2.
For such reasons, according to the eddy current testing probe reported in the prior art reference, since the output of the detecting coil 2 contains almost no noise component, it is possible to significantly improve the flaw detection accuracy.
However, the above-described eddy current testing probe reported in the prior art reference has a problem that the output of the detecting coil 2 still contains a noise component for reasons explained below.
FIG. 5 is a schematic view for explaining the state of a magnetic field generated around the eddy current testing probe reported in the prior art reference. As shown in FIG. 5, in the eddy current testing probe, since a solenoid coil having a short length relative to the coil diameter is often used as the exciting coil 1, the magnetic field generated by the exciting coil 1 contains not only a magnetic flux perpendicular to the surface of the test material T, but also a magnetic flux curved to the outside of the exciting coil 1 as the distance from the exciting coil 1 in the center axis direction thereof increases.
Accordingly, inside the detecting coil 2, the magnetic field in a direction crossing the detecting coil 2 increases as the distance from the exciting coil 1 increases, and therefore the noise component corresponding to a change in lift-off is contained in the output of the detecting coil 2.
On the other hand, compared to the case where the longitudinal direction of flaw and the detecting coil 2 are parallel, when the longitudinal direction of flaw and the detecting coil 2 are not parallel, the output of the detecting coil 2 decreases. Further, when the longitudinal direction of flaw and the detecting coil 2 are perpendicular, there is a problem that the flaw is undetectable and the flaw detection accuracy is low.
It is an object of the present invention to provide an eddy current testing probe capable of enabling a detecting coil to contain almost no component crossing the detecting coil, of a magnetic field generated by an exciting coil, in the inside of the detecting coil and reducing the noise component corresponding to a change in lift-off, contained in the output of the detecting coil, by positioning the detecting coil obtained by winding a conductor in the shape of a polygon such as a triangle and a pentagon so that one side of the polygon is placed on the exciting coil side and the vertex opposite to the one side is placed apart from the exciting coil.
Another object of the present invention is to provide an eddy current testing probe capable of detecting a flaw in a stable manner, irrespective of the direction of the flaw, by positioning a detecting coil whose center axis is in a direction crossing the center axis direction of an exciting coil on the center axis of the exciting coil and rotating the detecting coil about the center axis of the exciting coil.
Still another object of the present invention is to provide an eddy current testing probe capable of detecting the longitudinal direction of a flaw and accurately detecting the properties of the flaw, including the direction of the flaw relative to the test material, by detecting the rotation angle of the detecting coil.
Yet another object of the present invention is to provide an eddy current testing probe capable of detecting a flaw in a stable manner, irrespective of the longitudinal direction of the flaw, by positioning a plurality of detecting coils whose center axes are in a plurality of different directions respectively crossing the center axis direction of the exciting coil.
A further object of the present invention is to provide an eddy current testing probe capable of obtaining an output voltage indicating the properties of flaw most accurately by selecting the maximum output voltage from the output voltages of a plurality of detecting coils, and thereby improving the flaw detection accuracy.
An eddy current testing probe according to the first aspect of the present invention is an eddy current testing probe comprising: an exciting coil; and a detecting coil whose center axis is in a direction crossing a center axis direction of the exciting coil, wherein the detecting coil is composed of a conductor wound in a shape of a polygon, and positioned by placing one side of the polygon on the exciting coil side and placing a vertex opposite to the one side apart from the exciting coil.
In the state where a side surface of the exciting coil on the side opposite to a vertex of the detecting coil at a distance from the exciting coil (the vertex in a position closer to the inside center of the exciting coil relative to one side of the polygon placed on the exciting coil side) is placed to face a test material, by causing an alternating current to flow in the exciting coil, it is possible to detect a flaw on the surface of the test material. In the detecting coil in the shape of a polygonal ring whose top part space becomes narrower as the distance from the exciting coil in a direction crossing the exciting coil increases, it is possible to make the space inside the detecting coil smaller as the distance from the exciting coil increases, thereby enabling the space inside the detecting coil to contain almost no magnetic field in a direction crossing the detecting coil.
FIGS. 6A and 6B are explanatory views for explaining the direction of the magnetic field in the vicinity of the detecting coil. As shown in FIG. 6A, in the detecting coil 2 in the shape of a quadrangular ring, the distance between two sides placed in the direction crossing the exciting coil 1 is uniform irrespective of the distance from the exciting coil 1. In contrast, as shown in FIG. 6B, in the detecting coil 2 in the shape of a polygonal ring like a triangle ring with the narrowed top part space, the distance between two sides placed in the direction crossing the exciting coil decreases as the distance from the exciting coil 1 increases. Therefore, in the space inside the detecting coil 2 in the shape of a quadrangular ring with four equal angles, in the outside space of a portion distant from the exciting coil 1, a large magnetic field in a direction crossing the detecting coil 2 is contained. On the other hand, in the detecting coil 2 in the shape of a polygonal ring like a triangle ring with the narrowed top space, since the inside space becomes smaller as the distance from the exciting coil 1 increases, almost no magnetic field in a direction crossing the detecting coil 2 is contained in the space inside the detecting coil 2. The magnetic field in the direction crossing the detecting coil 2 induces a voltage between the terminals of the detecting coil 2, and the strength of such a magnetic field varies according to a change in lift-off. Therefore, the induced voltage also changes according to a change in lift-off, causing a noise component. Accordingly, in the eddy current testing probe of the first aspect, it is possible to reduce the noise component corresponding to a change in lift-off, contained in the output of the detecting coil 2.
An eddy current testing probe of the second aspect of the present invention is based on an eddy current testing probe comprising an exciting coil and a detecting coil, and the detecting coil is positioned on the center axis of the exciting coil so that a center axis of the detecting coil is in a direction crossing a center axis direction of the exciting coil and the detecting coil is rotatable about the center axis of the exciting coil.
The detecting coil whose center axis is in a direction crossing the center axis direction of the exciting coil is positioned on the center axis of the exciting coil, and the detecting coil is rotated about the center axis of the exciting coil. When the detecting coil and the longitudinal direction of a flaw become parallel to each other, the maximum output voltage is generated in the detecting coil. The output voltage generated at this moment accurately indicates the properties of the flaw. Therefore, by obtaining this output voltage, it is possible to detect a flaw accurately and detect a flaw in a stable manner irrespective of the longitudinal direction of the flaw.
In the eddy current testing probe of the second aspect, the rotation angle of the detecting coil is detected. During one rotation of the detecting coil, when the maximum output voltage is generated, the detecting coil and the flaw are parallel. By obtaining the rotation angle of the detecting coil at this moment, it is possible to detect the longitudinal direction of the flaw and accurately detect the properties of the flaw, including the direction of the flaw relative to the test material.
An eddy current testing probe of the third aspect of the present invention is based on an eddy current testing probe comprising an exciting coil and a plurality of detecting coils, and the respective detecting coils are positioned on the center axis of the exciting coil so that center axes of the detecting coils are in a plurality of different directions respectively crossing a center axis direction of the exciting coil.
The plurality of detecting coils whose center axes are in a plurality of different directions respectively crossing the center axis direction of the exciting coil are positioned on the center axis of the exciting coil. Consequently, irrespective of the orientation of the longitudinal direction of a flaw, it is possible to detect the flaw with a detecting coil which is substantially parallel to the flaw. It is thus possible to detect a flaw in a stable manner, irrespective of the longitudinal direction of the flaw.
In the eddy current testing probe of the third aspect, the maximum output voltage is selected from the output voltages of the plurality of detecting coils. A detecting coil generates the maximum output voltage when the detecting coil becomes parallel to a flaw. Accordingly, the detecting coil generating the maximum output voltage is in a position more parallel to the flaw than the other detecting coils, and this output voltage indicates the properties of the flaw more accurately than the other output voltages. Therefore, a selected output voltage accurately shows the properties of flaw, and such a structure can improve the flaw detection accuracy.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.